Gochujang is a spicy Korean condiment that's made from fermented soy beans, glutinous rice flour and chili peppers. It's a blood red paste that typically comes in jars or plastic tubs and has a unique flavour that you really can't substitute with anything else. This should be widely available at almost any Asian grocery store.
Daepa are a type of scallion that are much longer and thicker than normal scallions, but they're not quite as fat as leeks. You should be able to find them in Korean markets as well as Japanese groceries where they go by the name of Tokyo Negi. If you can't find them, 3 regular scallions will work just fine.
Garae tteok are cylindrical rice cakes, similar to mochi, but they are much more dense and will retain their shape even after cooking for some length. Unfortunately there really isn't a substitute (mochi will have a much softer texture), if you have an Asian or Korean grocery near you they carry it.